FRACTURE-RESISTANCE OF SIC-FIBER-REINFORCED SI3N4 COMPOSITES AT AMBIENT AND ELEVATED-TEMPERATURES

The crack growth behavior in unidirectional SiC-fiber-reinforced Si3N4 -matrix composites fabricated in our laboratories was investigated as a function of fiber volume fraction and temperature. Both the stress-i ntensity factor and an energy approach were adopted in the characteriz ation of the crack growth behavior. Crack resistance increased with cr ack extension (R-curve or T curve) as a result of bridging effects ass ociated with the intact fibers. Large-scale bridging was observed, and was considered in the determination of the R-curves. Temperature and fiber volume fraction affected the crack propagation behavior. At room temperature a single crack was initiated at the notch tip; it then br anched and delaminated upon further loading. In contrast, at 1200 degr ees C, little crack branching was observed. Increasing fiber volume fr action increased the degree of crack branching. Temperature and fiber volume fraction also affected the R-curve behavior. Raising the temper ature to 1200 degrees C did not significantly degrade the room-tempera ture R-curve effect. Increasing the fiber volume fraction from 14% to 29% substantially enhanced the toughening effect and the R-curve behav ior.